Heated vessel for corrosive fluids

ABSTRACT

An externally heated process vessel useable with corrosive fluids. The vessel is formed by a thin-walled container of a plastic material that is non-reactive with respect to the solution, and a jacket surrounding the container and in intimate contact therewith. The jacket is constituted by an assembly of metallic plate sections on whose outer walls are mounted heating elements. The joints of the section are joined to each other or to the plastic container by resilient means which render the jacket expansible to allow for differences in the thermal characteristics of the inner container and the outer jacket.

United States Patent [191 Layton HEATED VESSEL FOR CORROSIVE FLUIDS [75] Inventor: Howard M. Layton, Pound Ridge.

[73] Assignee: Interlab,1nc., Danbury, Conn. [22] Filed: Aug. 22, 1973 [211 App]. No.: 390,330

[52] US. Cl 165/169, 136/161, 219/435 [51] Int. Cl. F28b 3/12 [58] Field of Search 165/81, 169; 219/226, 242,

219/3, 258,297, 304-306, 311, 335-338, 435, 438, 528; 220/2, 52, 126, 122; l36/l 6l.1, 161; 62/530 [11] 3,834,459 [451 Sept. 10, 1974 3,649,366 3/1972 Jordan et al. 136/161 Primary Examiner-Manuel A. Antonakas Assistant ExaminerTheophil W. Streule, Jr.

[5 7] ABSTRACT An externally heated process vessel useable with corrosive fluids. The vessel is formed by a thin-walled container of a plastic material that is non-reactive with respect to the solution, and a jacket surrounding the container and in intimate contact therewith. The jacket is constituted by an assembly of metallic plate sections on whose outer walls are mounted heating elements. The joints of the section are joined to each other or to the plastic container by resilient means which render the jacket expansible to allow for differences in the thermal characteristics of the inner container and the outer jacket.

7 Claims, 2 Drawing Figures 1 HEATED VESSEL FOR CORROSIVE FLUIDS BACKGROUND OF THE INVENTION This invention relates generally to heated vessels useable with highly corrosive solutions, and more particularly to a hybrid vessel for this purpose, the vessel having an inner shell of non-reactive plastic material and an outer metallic jacket.

In fabricating printed circuit assemblies and microelectronic substrates containing integrated circuits, among the steps involved are etching, plating and photoresist processes. In carrying out these procedures, it is necessary to immerse the work in a variety of solutions which are maintained at elevated temperatures in a range of about 50 C. to 100 C. Because these solutions are highly corrosive, the process tanks for containing the solutions cannot be formed of metals that are ordinarily acceptable for processing. Etchants of the type used in producing integrated circuits will attack all but most costly types of corrosion-resistant metals and alloys.

There are a number of plastic materials which are capable of withstanding attack from highly corrosive solutions and have sufficient structural strength to serve as a process vessel. But because the thermal conductivity of such plastics is relatively poor, one cannot effectively raise the temperature of the chemical solution by applying heat to the exterior of the plastic vessel, for the vessel thermally insulates the solution from the external heat source. And since plastic is a poor heat conductor, heat applied to one area of the plastic vessel tends to overheat this area while other areas remain relatively cool, thereby producing so-called hot-spots, which are undesirable.

It is for this reason when using plastic process vessels, that heating of the solution contained therein is usually accomplished by means of plastic or quartz-encased heating elements immersed in the chemical solution itself. But here too one encounters the interrelated problems of corrosion and thermal conductivity, for there are very few materials which are adequately resistant to the hydrofluoric acid solutions commonly used in microelectronic processings and yet are sufficiently conductive thermally to serve as a protective casing for a heating element.

In situations where the use of immersion heaters is precluded because of the nature of the chemical solution or by other factors, one often employs a doubleboiler technique wherein a thin-walled plastic vessel is held within a larger vessel, the space therebetween being filled with oil or water. Thus the corrosive process solution contained in the inner vessel is heated through the thin plastic wall thereof by the transfer of thermal energy from the surrounding liquid bath. This bath, in turn, is heated by immersion heaters. Alternatively, if the outer vessel of the double-boiler is of metallic construction, heating is effected by heating elements bonded to the exterior thereof.

One advantage of the double-boiler approach is that the nature of the bath fluid (water, oil, etc), can be selected to match the maximum permissible operating temperature of the corrosive fluid of the inner vessel.- For example, in an unpressurized system using a water bath, the water temperature cannot go above 212F. But double-boiler systems are cumbersome and have an awkward configuration.

Another approach to the problem of containing a heated corrosive solution involves a lined vessel in which a metal container is provided with an inner layer of plastic or glass material internally bonded to the wall of the container. The inner coating mustbe thick enough to avoid porosity and to protect the metallic container from chemical attack. On the other hand, the non-metallic coating must be thin enough to permit an adequate transfer of thermal energy from external heating elements to the processing solution. The lined vessel approach is more or less successful, depending on the extent to which one is able to reconcile the need for a non-permeable liner with the requirement for reliable control of the plastic-to-metal interface temperatu're.

In order to attain a more reliable distribution of thermal energy and lessen the risk of porosity, flexible blanket heating elements of the silicon-rubber insulated type may be bonded to the exterior surfaces of a thinwalled plastic vessel. By appropriate design of these heating blankets and by the use of high-temperature plastics in the construction of the vessel, an adequate quantity of thermal energy may be developed and transferred through the wall of the vessel to the processing solution without exceeding the allowable temperatures at the interface of the blanket and the vessel wall.

In practice however, it is difficult with this arrangement to achieve an entirely uniform watt density. Moreover, the heating elements change their characteristics with age, as a consequence of which local, high temperature regions often develop in time and these regions eventually cause damage to the process vessel.

SUMMARY OF THE INVENTION In view of the foregoing, it is the main object of this invention to provide a hybrid process vessel suitable for heating corrosive solutions to be used in microelectronic fabrication procedures and in other processes requiring heated corrosive fluids, the hybrid vessel being constituted by a thin walled, plastic inner container and a metallic outer jacket surrounding said container and serving as a heat distributor.

More specifically, it is an object of the invention to provide a hybrid vessel of the above-type wherein the jacket is composed of an assembly of metallic plate sections which are maintained in intimate contact with the plastic vessel by means of resilient articulated corner joints or by means of resilient bonding between the metal plate surfaces and the plastic vessel walls, thereby allowing for differences in the thermal characteristics of the container and jacket and leaving each free to expand and contract independently.

A significant aspect of the invention resides in the fact that the jacket surrounding the container is provided with heating elements attached to the sections, the jacket'functioning as a heat distributor to effect a substantially uniform distribution of the thermal energy applied to the container and to avoid the formation of undesirable hot-spots.

Also an object of this invention is to provide a hybrid vessel which is durable, efficient and reliable in operation and which may be manufactured and assembled at relatively low cost.

Briefly stated these objects are attained in a hybrid vessel formed by an inner container and a jacket, the

inner container being fabricated of a thin-walled plastic which is non-reactive to the corrosive solution, the jacket being defined by an array of metal plate sections whose joints are resilient to conform the sections to the walls of the container and to maintain the plates in intimate contact with the walls regardless of temperature changes that bring about differences in the relative dimensions of the container and jacket.

OUTLINE OF THE DRAWINGS DESCRIPTION OF THE INVENTION In the design of a process vessel one must take into account the expected temperature excursion to which the vessel will be subjected. Thus a vessel in which the 7 liquid therein is to'be heated from room temperature to a level of 50 C. can be said to undergo a limited temperature excursion, whereas if the liquid is to be heated to a level of 100 C., then the temperature excursion is large.

For a limited temperature excursion, one may use a vessel formed of a non-reactive plastic material having a metallic outer coating. This coating may be formed by bonding metal foil to the outer surface of the vessel or by plating this surface with a layer of copper or a similar metal of high thermal conductivity. Heating elements are then attached to the metal coating which acts as a heat distributor so that the heat is spread uniformly about the vessel and is transferred tothe corrosive fluid contained therein. Instead of using metal foil or a plating, a silverepoxy or other compound of high thermal conductivity may be employed as a heat distributor.

One difficulty with a metal-walled plastic vessel of the above-described type is that when a large temperature excursion is encountered in the process, the differences in the thermal expansion of the plastic vessel and the outer metal coat bonded thereto are such as to split or fracture the coating, thereby destroying the integrity of the thermal distribution system.

As shown in FIGS. 1 and 2, the hybrid vessel includes a thin-walled plastic container 10 whose open mouth is surrounded by a flat lip 11. The cross section of the container is generally square so that the container is defined by four side walls and a bottom wall. The container is molded or otherwise formed of a suitable high temperature, non-reactive plastic material of high strength, such as a polycarbonate, a polypropylene or Teflon material.

Surrounding container 10 below lip below lip 11 is a metal jacket constituted by an assembly of articulated metal plate sections 12. These sections are formed of a metal of high thermal conductivity, such as aluminum or copper. In order to conform to the shape of the container, the jacket assembly consists of five flat sections, four of which lie against the side walls of the container and the fifth against the bottom wall thereof. In the event the inner container has a curved wall, the sections are then similarly curved to conform the jacket to the container.

In order to articulatethe sections, the four margins of each plate section are folded .to define marginal flanges 12A, 12B, 12C, and 12D. The complementary flanges of the array of sections are clamped together by coupling screws 13 which are biased by helical springs 14, preferably of steel. The constraining spring pressure is adjusted by the screws to force the sections into intimate contact with the related side and bottom walls of the inner container.

Attached to the exterior surface of each section is a flat heating element blanket 15 whose dimensions match the area bounded by the flanges 12A to 12D. Overlying the blanket is a reflective foil backing 16, serving to direct radiated heat toward the container wall. Thus each section is provided with its own heating element.

Because the clamped joints are resilient, the jacket is capable of expanding or contracting so that the jacket sections are always maintained in intimate contact with the container walls regardless of differences in the thermal expansion characteristics of the inner plastic container and the outer metallic jacket.

A further improvement in performance may be effected by the use of a thermally conductive flexible epoxy layer 17 or other bonding agent between the metal plates and vessel walls, especially at the corners to fill in the interstices and transfer thermal energy to areas not covered by the clamps themselves. Moreover such flexible bonding agent may serve effectively as an alternative to the articulated resilient corner joints as a means for maintaining the metallic heating jacket in appropriate thermal contact with the plastic vessel. In practice, the gaps between the jacket and the inner vessel may be filled with crushed metal foil.

It is noted that a very high thermal conductivity is required for the plate sections but not for transferring energy to and through the vessel walls. The reason for this is that the primary requirement is to maintain uniformity of temperature throughout the jacket and at all areas which interface with the plastic vessel walls. It is this facet of the temperature control requirement that protects the system while still permitting adequate thermal transfer through the vessel and into the processing solution. The moderate degree of thermal conductivity availablein most non-rigid epoxy and other compounds is quite satisfactory for service as a coupling medium between the metal jacket and the plastic vessel walls.

With a hybrid vessel system constituted by the plastic container and a metal jacket in initimate contact therewith, the thermal energy applied to the outside of this assembly tends to become uniformly distributed throughout the entire metallic area of the jacket, rather than to develop localized hotter regions where the heating element watt density has increased.

In practice, the metallic outer jacket of the process vessel may be extended to form an even more efficient heat-sink by attaching to it an external aluminum skirt or enclosure or other external metal foil wrapping thermally bonded to the metallic outerside of the vessel assembly and having a substantial proportion of its surface area exposed to the ambient air.

Although forming no part of the present invention, the hybrid vessel also includes a temperature-control sensor 19 which is immersed in the fluid, and plate sensor 19 is mounted on the heating element blanket 15.

While there has been shown a preferred embodiment of the invention, it will be appreciated that many changes may be made therein without departing from the essential spirit of the invention.

What I claim is: 5 l. A heated process hybrid vessel useable with corrosive solutions, to be heated to a level in excess of 50 C said vessel comprising:

plates.

2. A vessel as set forth in claim 1, wherein said container has a rectangular configuration defined by four side walls and abottom wall, and said assembly is composed of four side plates and a bottom plate.

3. A vessel as set forth in claim 1, wherein said plate sections are folded to define marginal flanges which are joined together by connecting springs which serve to clamp said plates together.

4. A vessel as set forth in claim 1, wherein said plates are bonded to said container walls by a resilient bonding agent.

5. A vessel as set forth in claim 1, wherein said elements are in the form of heating blankets.

6. A vessel as set forth in claim 5, wherein each element is covered by a reflective foil backing.

7. A vessel as set forth in claim 1, wherein said plates are formed of aluminum. 

1. A heated process hybrid vessel useable with corrosive solutions, to be heated to a level in excess of 50* C said vessel comprising: A. a thin-walled inner container formed of a high temperature, high strength plastic material that is non-reactive with respect to said solutions, B. a heat-distributing jacket surrounding said container and constituted by an assembly of articulated metallic plate sections in intimate contact with the walls of said container, said sections being resiliently joined to each other at their margins to render the jacket expansible to allow for differences in the thermal characteristics of the inner container and the outer jacket; and C. heating elements mounted on the exterior of said plates.
 2. A vessel as set forth in claim 1, wherein said container has a rectangular configuration defined by four side walls and a bottom wall, and said assembly is composed of four side plates and a bottom plate.
 3. A vessel as set forth in claim 1, wherein said plate sections are folded to define marginal flanges which are joined together by connecting springs which serve to clamp said plates togetHer.
 4. A vessel as set forth in claim 1, wherein said plates are bonded to said container walls by a resilient bonding agent.
 5. A vessel as set forth in claim 1, wherein said elements are in the form of heating blankets.
 6. A vessel as set forth in claim 5, wherein each element is covered by a reflective foil backing.
 7. A vessel as set forth in claim 1, wherein said plates are formed of aluminum. 